Hereinafter, a conventional cathode ray tube apparatus used for a computer display monitor, a television receiver and the like will be explained referring to FIG. 1. FIG. 1 is a view showing a cathode ray tube apparatus of this invention. However, the general construction of a cathode ray tube apparatus of this invention is same as that of the conventional cathode ray tube apparatus. Therefore, a conventional cathode ray tube apparatus will be explained referring to FIG. 1. FIG. 1 is a partially-sectional side view showing a cathode ray tube apparatus, with the upper portion of FIG. 1 being a sectional view showing a cathode ray tube apparatus. In FIG. 1, a cathode ray tube 1 comprises a panel 2 and a funnel 3 connected to the panel 2. Inside the panel 2, a phosphor screen (not shown in FIG. 1) is provided and a shadow mask (not shown in FIG. 1) is provided. Electron guns (not shown in FIG. 1), which are in-line aligned, are provided inside of a neck portion 4 of the funnel 3.
In FIG. 1, 11 indicates a deflection yoke that deflects an electron beam to the horizontal direction and to the vertical direction. 12 indicates a central processing unit (CPU) that controls the purity and the convergence in the mid section of the picture and comprises 2P (a magnet that generates a double pole magnetic field), 4P (a magnet that generates a four pole magnetic field) and 6P (a magnet that generates a six pole magnetic field). 2P, 4P and 6P are not shown in FIG. 1. 13 indicates a reference line. The reference line is a virtual line and is reference of the tube axial direction of the cathode ray tube. An electron beam actually is emitted from the side of the electron gun, however, it is equivalent to an electron beam is emitted from the reference line 13 and extends at an angle with respect to the longitudinal axis. "A" indicates a deflection angle. When a cathode ray tube apparatus has a 90.degree. deflection angle and comprises a round type panel having a small curvature, it is comparatively easy to correct a distortion of the picture in the upper-and-lower side by a self-convergence system that corrects the distortion of the picture automatically with the deflection yoke 11.
Hereinafter, the reason for the above-mentioned easy automatic correction will be explained referring to FIGS. 5 and 6. FIG. 5 shows the relationship between a position of P along the axis of cathode ray tube and a magnetic field H which is generated by a deflection yoke. The magnetic field H indicates a ratio of the magnetic field in the position of P along the axis of cathode ray tube to the magnetic field in the whole area of the deflection area. The horizontal axis P is divided into three parts, a screen side part 5 to the right side of point "b", a mid section 6 between point "a" and point "b" and an electron gun side 7 to the left side of point "a", with respect to the contribution ratio of magnetic field to each property such as the convergence and the raster.
FIG. 6 shows the relationship between a position of P along the longitudinal axis of the cathode ray tube and a contributing ratio of magnetic field R to each property, such as the convergence coma 8, the convergence astigmatism 9 and the raster distortion 10. The contributing ratio R is the portion of the magnetic field H that influences each property. The relationships shown in FIGS. 5 and 6 are well-known. According to the relationship shown in FIGS. 5 and 6, when a length of the deflection coil is constant, the pin cushion distortion of raster is greatly influenced by the magnetic field at the side of screen side 5. Further, it is well-known that the pin cushion distortion of raster in the upper-and-lower side is greatly influenced by the magnetic distortion in the horizontal magnetic field. In addition, it is also well-known that the pin cushion distortion of raster in the right-and left side is greatly influenced by the magnetic distortion in the vertical magnetic field.
As above-mentioned, the pin cushion distortion in the horizontal magnetic field, especially at the screen side 5, is strengthened beforehand, and the size of the area of the horizontal magnetic field at the screen side 5 is made as small as possible. As a result, the automatic correction of the pin cushion distortion of raster in the upper-and lower side was conducted comparatively easily.
FIG. 7 shows a case in which the pin cushion distortion of raster in the upper-and-lower side is corrected automatically. In FIG. 7, the pin cushion distortion indicated by the broken line is corrected automatically to be the horizontal line indicated by the arrow.
However, in comparison with a panel of a conventional type, a recent panel such as the 2R type is flattened. Further, the deflection angle is enlarged to be 100 degrees or 110 degrees. The picture tube having the above-mentioned panel has a problem such that the pin cushion distortion of raster in the upper-and-lower side is strengthened further, and therefore it is difficult to correct the distortion automatically.
Accordingly, there are some methods to improve the above-mentioned problems. For example, as shown in FIG. 8, a method in which a magnet 22 is attached to the upper and lower parts of the opening portion of the deflection yoke 21, and a method as disclosed in publication of Japanese Patent Application Tokkai-Sho 59-3849 in which the size of the coil is miniaturized and a deflection center is shifted as close as possible to the neck portion of cathode ray tube to decrease an effective deflection angle of the electron beam, have been proposed.
However, even when the above-mentioned methods are employed, it is still difficult to conduct the automatic correction. Therefore the correction with the electrical circuit is required. In case of the correction with the electrical circuit which is conducted by controlling the deflecting current wave form, when the horizontal deflecting frequency is changed, the degree of the correction of the raster does not become to be optimum, or the convergence is changed, therefore, it is impossible to correspond with the multi-scan.